Selective calling system



March 2, 1954 J. K. KULANSKY SELECTIVE CALLING SYSTEM Filed Dec. 2l, 1946 4 Sheets-Sheet 1 r To l 0mm/rer ATTORN March 2, 1954 J. K. KULANSKY SELECTIVE CALLING SYSTEM 4 Smets-Sheet 2 N. lml

Filed Dec. 2l, 1946 kvm.

INVENTOR Jac/r /fend'd/ /fa/ans/ry BY ORNEYS ATT March 2, 1954 J. K. KULANSM 2,671,167

SELECTIVE CALLING SYSTEM Filed Dec. 21, 194e 4 sheets-spaans March 2, 1954 J. K. KuLANsKY SELECTIVE CALLING SYSTEM 4Sheets-Sheet 4 Filed DeC. 2l, 1946 INVENTOR. Jac/r /fenaa/ Kwam/ry l ATTORNEYS;

, XQ mum n M R6 W .Q iw Q@ NTM H) wswkc@ NTQ MSS RES Patented Mar. 2, 1954 2,671,167 sELEoTlvE CALLING SYSTEM J ack 'Kendal Kulansky,

to The 'Hammarlund Inc., lNew York, N. .York

Brooklyn, N. Y., assigner Manufacturing Company,

Y., a corporation of New Application December 21, 1946, Serial No. 71'ji'27 Cl. 'Z50-4i) -6 Claims.

'This invention relates to selective calling systems, and more particularly to systems oi a type Suitable for use with a central radio transmitter and a plurality of mobile receivers.

An object of the invention is to provide a system in which the receiver of any desired mobile station may be rendered operative under the control of the central station operator.

A further object of the invention is to provide a system in which the signaling frequency for selecting a particular mobile station will not form an `,appreciably large component oi the frequencies used for communication.

Another object of the invention is to provide a distinctive signal for all yunwanted mobile stations indicating that the system is in use, thus tending to prevent any unwanted mobile station from interrupting communications .between the central station and a called mobile station..

The invention also provides a positive lock-ouh which may be used if desired, supplementing the distinctive signal to unwanted stations ,by cutting off the transmitter control .circuit at each such station.

Another object of ythe invention is to provide a distinctive restoring signal for use at the con clusion of each communication which wiil indi cate at all mobile stations that the system is no longer in use and that any mobile station may call the central station without interference. This l restoring signal may also reset the transfmitter lock-out.

A further object of the invention is to provide response of either audible and visual signals or both at the called station with facilities for auto'- znatically restoring the signals upon answering the call and for likewise restoring the signals by transmission of the distinctive restoring signal, in the event that the called station should not answer.

A further object is to provide an audible signal which will be silenced remotely if the called station does not answer within a reasonable interval 'together with a visual -signal which will remain actuated until reset, thus indicating to the operator that an attempt was made to com municate with him while he was away from the station.

Other and further objects will become ap parent upon reading the annexed specification together with the accompanying drawing.

Referring to the drawings-.-

Fig. l shows a diagrammatic Yrepresentation of the facilities at the central station;

2 Fig. 2 shows a similar representation of the facilities at each mobile station;

Fig. 3 shows a schematic circuit drawing of the transmitting facilities of an embodiment of the invention;

Fig. 4 shows a schematic circuit drawing of the receiving and decoding facilities of an embodiment of the invention;

Fig. 5 shows a circuit drawing of a visual signaling arrangement for use with Fig. 4; and

Fig. 6 shows a circuit drawing of an arrangement for use with Fig. 4 which combines both audible and visual signals` ln Fig. 1, an audio-frequency mixer l-ii is interposed between the input of a radio transmitter (not shown) and the usual speech input and press-to-talk circuits. The mixer i-ll! permits the transmitter input to be derived from either the tone oscillator I-'QZ or from the speech input facilities without the necessity of switching, and without appreciable interference between these input circuits.

Tone oscillator l-2 delivers a frequency to the transmitter input which may be selectively received at the mobile stations and which will not be present in the communication frequencies in sufficient magnitude to cause interference with the operation of the selective signaling apparatus. Preferably, a tone frequency such as 6000 cycles may be used which lies above the frequency range required for `voice communication and which can be handled by most radio equipment with a minimum of modincation, if any modiiication at all should be required. For systems in which the radio apparatus uses frequency modulation, a signaling tone frequency of the order of 2600 cycles has been found to give good results. A frequency below the speech range may be used, but this involves slower signaling speeds and more bulky components in the apparatus used for filtering the tone frequency to separate it from the speech frequencies. y

The tone output of oscillator I-llZ is under control of pulsing dial i-DS and keying switch Pulsing dial l-lli may be of any well known type such as the device commonly used. in dial telephone systems, or it may take any other convenient form. Pulsing dial l-@S causes oscillator I-il? to deliver groups of uniformly spaced tone pulses, the number of pulses in each group oorresponding to one digit of the call number of the desired mobile station. These pulses are passed through mixer i-lli to the input of the central station radio transmitter for transmission to all mobile receivers.

At the conclusion of a communication with a mobile station, a reset signal is transmitted by operating reset button l-, which in turn initiates a single cycle of operation of reset timer` [-05. At the conclusion o each cycle oi operation, timer l-il is again in condition to be reoperated by subsequent operation of reset button l-.

Reset timer l-il, in the course of its cycle, first causes timer switch i-ll to control pulse generator l-ll to produce a predetermined number of pulses which in turn produce corresponding actuation of keying switch i-M causing tone oscillator l-l2 to deliver a corresponding number of tone impulses to mixer l-UL These impulses may have the same spacing and duration of the impulses produced by pulsing dial i-tt, but preierably they are shorter in duration and delivered at a faster rate than those produced by dial l-iiii within the speed limitations of the pulse-responsive apparatus of the system in order to minimize the restoring` time. At the conclusion of transmission oi the predetermined number of impulses, reset timer i-ll next causes dash generator l-il9 to control keying switch l-M to produce a prolonged tone impulse by tone oscillator i-2. The duration oi this impulse is sufficient to permit it to be readily distinguished from the pulsing impulses. The number of pulses immediately preceding the prolonged impulse is sufficient to bring all pulse-responsive apparatus of the system to a predetermined condition before the prolonged impulse is transmitted.

The apparatus utilized at each of the mobile A stations is shown diagrammatically in Fig. 2. A selective iilter 2 0! may be connected in the circuit of the audio output tube of the mobile receiver, or it may be connected in any other convenient portion of the receiver circuit where it will eiiectually select the signals transmitted by tone oscillator i-ll of Fig. l from the communication frequencies. The selected signals are ampliiied by amplier 2 9?. which may be provided with a gain control independent from the control which adjusts the receiver audio output level and the output of amplifier 2-92 is applied to tone-responsive relay 2-3. Relay 2-@3 controls circuits linking it both with pulsing time discriminator 2-0il and with release time discriminator 2435. Tone-responsive relay 2-63 also controls circuits conjointly with loudspeaker switch 2-59 which suppress operation of the loudspeaker circuits. These circuits are arranged to mute the loudspeaker, preventing the tone impulses from being obiectionably audible. This is especially desirable during transmission of the restoring impulses described below, since the loudspeaker of at least one mobile station will be in operative condition. Pulsing time discrimlnator Z-ll'controls the operation of restorable stepping switch 2-06 in accordance with the number of tone impulses received, but depending upon constructional details of stepping switch 2-95 pulsing time discriminator 2-011 preferably should not respond to the prolonged impulse transmitted under control of dash generator l-Gll. Release time discriminator 2-05 is responsive only to prolonged tone impulses under control of dash generator i-ti. Stepping switch 2-05 will always be brought to its most advanced position from any other position by the predetermined number of pulses produced by timer switch l-il, the predetermined number of pulses having been selected to be suiicient for this purpose. In this most advanced position, actuation of release time discriminator 2-05 by the prolonged pulse oi dash generator lwill cause operation of decoding relay 2 8? which will in turn cause operation of restoring magnet 2438. Upon operation of restoring magnet Z-l, stepping switch 2l returns to its initial position and is again prepared to receive calls. instead of a restoring magnet and depending upon the construction of the stepping switch Z-il, the switch may be caused to advance in the same direction beyond its most advanced calling position to reach its initial position.

Stepping switch 2-05 is arranged to select the desired mobile station by reaching its most advanced position. The stepping switches of unwanted stations never reach this position. This situation further requires that the sum of the digits of the call numbers of all mobile stations shall be the same and that this sum shall be the number of pulses required to bring each stepping switch from its initial to its most advanced position. The stepping switch contacts at the positions corresponding successively to the several digits of the call number of a particular mobile station are electrically arranged to produce no operation of decoding relay 2-01. In all other positions, if the switch remains in such a position for an interval appreciably longer than the interval between impulses, decoding relay 2-91 will cause operation of restoring magnet 2-(38 thereby restoring the stepping switch to its initial position. Upon being thus restored before all the digits of the call number have been transmitted, each such switch will lag behind the switch of a desired station and will fail to reach its most advanced position.

The connections to the stepping switch 2-06 which determine the individual call number of each mobile station are preferably included in 1" removable plug 2-12 or in some e switch 2-(39 places the receiving other similar device aording convenience in establishing a plurality of removable connections. This arrangement facilitates the replacement of selective equipment in the mobile units by permitting all selective units to be wired identically and interchangeably. In replacing a unit for maintenance purposes the replacement unit is installed and the code plug is transferred from the former unit to the replacement unit.

Any other convenient form of selective apparatus may be utilized which will respond selectively to the dialing in proper sequence of the digits of its call number.

At the selected mobile station, loud speaker facilities in fully operative condition. At each unwanted station, transmitter lockout 2-l is rendered effective thereby preventing all such stations from intertering with the communication taking placebotween the central station and the desired station. The existence of this condition is made known to the operator of each mobile station by operation of visual signal Z-H in conjunction with transmitter lockout 2-18 indicating that the system is busy. Loudspeaker switch 2-95 does not operate at the unwanted stations, thus avoiding annoyance which would otherwise be caused to the operators at the unwanted stations by hearing all communications emanating from the central station. At the conclusion of each communication, the central station operator operates reset button l-tl5 initiating the distinctive restoring signal thereby placing all mobile stations in condition to originate calls to the tial positive with respect current in triode 3-5l.

substantiall `in a direction to render the triode :3-3'9 cease to generate pulses.

drawing additional current from the anode supply and passing this through cathode vresistor 3-8 to maintain the cathode at a potento ground independently or any ow of current in the anode circuit. Upon receipt of each positive pulse, triode S- becomes conductive and effectively permits the ow of current from terminal S-l of oscillator 3-3 to produce a pulse of tone output.

Referring now to triodes 3-32 and 3-2il, the positive pulse which appeared on the anode of triode 3-32 and which passed along conductor 3-34 causing triode 3-35 to become conductive,

Valso passed along conductor S-i through blocking capacitor 3-5l and applied an increased positive potential to the grid of triode 3-51. Because the grid of triode 3 5! is normally maintained at the anode supply potential, this positive impulse had no effect. However, at the end of a time interval established by the constants oi resistors 'and capacitors associated with triodes arranged. Upon this return to a conductive conductive state. By a proper selection of circuit constants, this change can be made abrupt in nature and the circuits should preferably be so arranged. Under this return to a conductive condition the potential drop through resistor 3-33 suddenly increases resulting in a negative pulse along a conductor 3-49 to the grid of triode B-Si. This negative pulse stops triodes 3-33 and 3-39 by rendering triode 3-36 non-conductive and it also reduces the anode The reduction of anode current in triode 3-51 has the effect of reducing the potential drop across resistor 3-52 which in turn produces a positive pulse which is applied through capacitor 3-53 to the grid of triade -Sl.

This positive pulse, as in the case of triode 3-l5, produces a tone output from oscillator S-i, the arrangements of triodes -d and 3-54 being identical. As triode 3-Ei became less conductive, an increased positive potential was applied to the grid of triode 3-55 through the voltage divider circuit comprising resistors 3-56 and 3-51 and increasing the conductivity of triode 3-55. This increased conductivity in turn produces an increased potential drop across resistor 3-58 resulting in a positive pulse which is applied through capacitor 3-59 to the grid of triode i-iii tending to offset the which originated the cycle.

The time constants of the circuit elements associated with triodes 3-51 and 3-55 are so selected that the negative pulse along conductor '1i-d8 initiates a prolonged positive impulse applied to the grid of triode 3-54 which produces a vprolonged tone pulse at the output of oscillator 3-i3.

The operation of the reset circuit may be summarized as follows:

Closure of reset button 3-26 initiates 'a time cycle in the circuit of triodes 3-32 andl 3-29.

During this time cycle, capacitor 3-35 is charged 3-36 conductive. The conductive condition of triode 3-36 produces a series of pulses in the circuits of symmetrically arranged triodes rI-illl and 3-39. These pulses control the tone oscillator 3-l 3 and a series of pulses appears at its output at a rate which is determined by the circuit constants and which is selected to conform to the maximum speed of response of pulse responsive apparatus, in the '.'system At the expiration of the time interval established in triodes 3-32 and 3-29,. conduction through tube 3-36 ceases and triodes 3-38 and the action of pulsing negative pulse Anims time, a

starting pulse is applied to the circuit of triodes 3-51 and 3-55 which in turn initiates a subsequent timing cycle between these two triodes, the duration of this latter cycle being suiicient to produce a prolonged tone impulse or dash which appears 'at the output of oscillator 3-l3 by virtue of a conductive condition established in triode 3-54 during this prolonged impulse.

At each mobile station, receiving equipment is provided which may take the form illustrated in Fig. 4. Radio receiver 'li-0| is provided with an output tube 4-612 of which only the anode has been shown dotted. The receiver may be of 'any conventional type and the details of its construction do not form a part of this invention. The anode oi the output tube 4-02 is energized through the primary winding o f output transformer 4-03 and through iilter ffl-04. Filter 4-04 is sharply tuned to the frequency of tone oscillator 3-I3 of Fig. 3. This frequency is preferably of some value removed from the mean frequency of the communication frequency components. In practice, good results have been obtained using a frequency of 6.0 kc. for the tone oscillator and also with a frequency of 2.6 kc.

Both of these frequencies are well removed from the relatively increased energy components which, in the case of voice communication, lie in the neighborhood of 1000 cycles. Because of the sharp tuning of lter 44M relatively little interference is produced with the response of output transformer ll-Q3 at frequencies other than those in the immediate range of the frequency of the tone oscillator. The output of filter 4-64 is connected to the control grid of the pentode 4-95 which has its screen and suppressor grids connected in conventional manner.

It may be desirable to provide separate ampliilers for the audio output and the tone output of the receiver with individual gain controls. The details of such an arrangement may take any well known and conventional form. For this reason they are not shown in the drawings. The use of separate gain controls permit the volume delivered by loudspeaker 4-d2 to be varied to deliver any desired level appropriate to the listening conditions while the tone output is maintained at a relatively fixed level determined to care protected against undue arcing by a Aspark suppressor comprising capacitor l-IB and resistor -H. A selector switch -l2 is illustrated as being provided with two banks of ten terminals. A stepping magnet 443 is disposed upon each deenergization thereof to cause counter clockwise rotation of the wiper arms 4-i5 and 446, the amountv or motion being one bank contact for each complete impulse applied to the stepping magnet. Switches of this'type are well known and the details of its construction will not be set forth more than is necessary to insure that a switch may be selected which will have the requisite characteristics for correct operation of the circuit. The switch may be reset to its normal position by energization of reset magnet d-ic', the switch being shown in its normal position in the drawing. In all positions eX- cept the most advanced position contact -I 8 engages' contact 4l9. Whenever switch "ll-I2 reaches its most advanced4v position, contact ll-l disengages contact. 4.-|9 and engages contact, 41-20 in. preparation for recepit of the prolonged iinu pulsey of the restoring signal. The, action, oi the advanced position contacts 448, 449,y L29 constitutes an electromechanical pulsing gate permitting energization of the reset magnet 4-17 from the make contact @-39 ci relay lie upon receipt the prolong-ed` pulse oftiie restoring signal. Each, switch will be stepped.' around, to its, most advanced position bythe preiinnner-ypulses of the restoring Signal. Accordingly, each switch will be conditioned, for restoration by the prolonged. pulse, which is of sufcient dii-ration. to. insure operation or relay d.-24,., it should; be: noted that. because switch 44,2; advances upon. deeneraiaationA of; its step manet niet the cir-A cuitof conductor t-,2e will be deenergiaed as, the. advanced position contacts are.. actuated unen. reaching the nnal position and; interrupt the cire cuit of the. stepping; ma 4.43 by opcninsconf taots 448., lit-i9.

Upon receipt of. a. tone impulse, relay este. closes contacts e101, 4-09, energizing sten magnet 443; through a circuit traceable from battery .through step magnet 4-13.. conductor @ein closed con: tacts 4.122-, 4.-.23 of slow operate rel-ay' 4-24, ad vanced position; contacts 448. dele. conductor 4-25, Contact 4-09 and, armature 4F02? to A. single impulse thus. advances switch del?. from its normal to its niet position.

At theA end of the pulse, armature d-t.- transf fers from contact 4-@9 to contact i-, establishine a circuit via conductor 41-26 to wiper arm. dei-6. which is now .engagement with the rst bankv contact. Connection to each oi the rst nine bank; terminals associated with wiper arm t-.IS isestablished; through a. code chane-v ine plus 442;?. This plus. is readily removable and all connections which determine. the code number to which a particular mobile station will. be responsive are established within the plus.` This permits the apparatus of Fis.. e to be identical in all; respects for each oi the. mobile stations.- with the exception of, the. code changing. plus..

The use of suc-h plus; connections greatly facilitates maintenance. A, receiving unit as, shown Fig. 4, and Fig. or Fig;` 4 and Fia, 6,. may be :ree moved from a mobile station, where. it is usual-ly diiipult to Work on the. equipment, and may be replaced by another identical unit..` The. code.: chanoine: plus. 412i is tnen transferred from the former unit to the replacement uni-t, reestabe lisbine the sar-ne code number for the mobile stae tion. In, the. illustration of. Fig., e.. codev chang-.- ing plus, de?? is snow-n. wired ior the four die-itv code. number 12.34.,-

In reaching the first. selector switch. position wiper arm 4.|6 finds no connection. at this rst. terminal in. the illustrative example of Fis. 4. .At all, other mobile stations having a code number whose first digit is other than one, a oonnecn` tionwill be established through, the code chaine-- ing plus' and. .conductor lL28. to Slo-w operate ree lay 4,-24.

The operate time oi slowoncrate relay-4112.4 exceedsthe maximum pulse duration but isshorter than the interval between consecutive Ydig-its of a code number. Therefore, during .the interval between digits-, at all stations. Whose code number does not begin with digit 1iv relay 4,-24,

' will, operate,v closing .the circuit to reset magnet 4|'l through conductor 4.-?29 which ,resets ail such selector switches to their normal positions. At all mobile stations in. which the iirst digit of the codenumber is l. no such. resetting' Seli-5. by ou action will. occur and; the sole-tor switches will remain their rst positions Considering the operation of. Fis.. a, in ooniunotion with. Fie.. in reach-ine or in bassine the first, position.. at all. mobile stations wiper 4.45 establishes a circuit. from around conductor 'letzt and e-it and; Winding,- oi. relay- Eeul through resistor Seti? to battery. rEhis establisnes a; loo-kine circuit through arm 7ure t-eliand contact teilt which circuit is tiieseaiter independent; oiV iurther Contact between wiper. arm #mit and its first bant. terminal which is around- Upon` lotions 1in-, armature 5-03. breaks its normally closedt acts artt dinos lamp 1Tl` or 5i6r in, series, with the ianio. This dininiina action indicates to the op.- erator oi: each mobi-le station that the system is .bu-sy.. iii preferred., a. lamp b-t'l may be pro vided is normallyextinguished and which. is energized in, multiple with the Winding of, relay 5-01 by Conductor 5,.-331. time, armature -ie of relay 'ti-9i breaks its normally closed Contact 5-l0 interrupting ,the circuit. oi prestato-talle button. bei! thus renderine, it impossible to. operate. transmit-receiverelay- 54.2.. Transmit-receive relay 5 -12 may be ariane-ed in. any .desired conventional and Well known manner such that upon onere-ization. .of its. winding the mobile transmitter is brouehtinto active condition, the receiver beine out olii while the trans itter is in operation. Normallyi with the winding,v oi itransmitereceive relay i2- .deencrgizedi the. reoeiyer is in. opera# ndition and the transmitter is. inactive.v Relay Seli include antenna transfer iacili` ties and. platepower transfer facilities and it may also comprise dynamotor control circuits for the transmitterplate supply.` Unless relay 5-12 can be. oper-atado the mobile transmitter cannot be used. 'lhusl armature bete and contact 5-10 in. breakina- .tne circuit oi press-to-.tal-is; button -l-i., locks out the mobile transmitter and prevents interior-ins use of .the transmitters of. un.- Wanted .stations-v As .the next digits of the code number are dialed, each selector isreset` to, normal` if it rests between dieits on a. bank.; terminal associated. with wiper c l t which is connected, through code. chanoine, plus. lieti to. conductor 4-28. In

- this manner; cach selectorL except the selector every other station it Will remain at. some inl' termediate. position.. Obviously, a selector switch having more than lo oli-normal positions may be usedJAY in which. case, the. sum or" the digits of every code number will be equal to the number of positions oi the selector switch disposed between the normal position and the, most, ad vanced, position.

A loudspeaker 4-i2f is provided with a, switch 4 5@ permitting it to be. out or out of service, at will. Connected in. multiple with loudspeaker 4,42, is thereceiver ll5l and series resistor il-t. Receiver -.l isl preferably part ofthe handset (not shot/111.2 used for two way communication. Included in this same handset may be a transr1 mitter of suitable design adapted for use with the radio transmitter of the mobile station .and

li These arrangements are well known in the communications art and will not be described in further detail. Series resistor 4-52 serves to reduce the audio frequency level at the loudspeaker to a level suitable for use with receiver 4-5l which is applied directly to the ear. Ii desired, an independently adjustable potentiometer may be used instead of series resistor 4-52 in conventional and well known manner.

When the selector switch 4-l2 at the desired station reaches its most advanced position, receiver 4 5! becomes operative, together with loudspeaker Q32 provided switch 4-58 is closed. This circuit is established from ground at one side oi the secondary winding of audio output transformer ll-S, through series resistor ll-Z and the operating winding of receiver ll-l along conductors 4-3l and 5 3! to the tenth terminal of the bank of selector switch 4 2? associated with wiper arm -IG, through wiper arm A-lt, conductor lll-2E and contacts A-t, -il'l of relay lto ground.

At the same time, the operating winding of relay -Gl is short circuited via conductors ll-33, 5-33, wiper arm fil-l5 and its associated tenth bank terminal, and through conductors 6F32,-

5-32. The ilow of excessive current is prevented by resistor 5-02. As soon as relay 5 0! releases, the circuit of its operating winding is deenergized by the opening oi contacts S-ilS, E-M. Closure of contacts 5-93, 5-03 short circuits resistor 5-06 restoring lamp 5-65 to full brilliancy.- If busy lamp 5-87 is provided, it is extinguishedby the deenergization of the operating circuit of relay S-lll. Closure of contacts S-s, -lt reestablishes the circuit oi press-to-talk button E-ll permitting normal use of the transmitter.

The desired mobile station is now conditioned for communication with the central station. At the other mobile stations, the selector switches have failed to reach their most advanced position. Consequently no loudspeaker circuit is established through wiper arm il-lG and relay -l remains locked in keeping the press-to-talk circuit disabled and maintaining a busy such as dimming of lamp At the conclusion of tion. As previously explained, the signal comprises a series of pulses pulse. These pulses are suiicient in number to bring all selectors to their most advanced positions by the time that the prolonged pulse is transmitted. The prolonged pulse establishes a circuit from ground through armature 4-91 of relay li-G6, conductor 4-25, advanced position contacts Ii-lll, 4 2@ and conductor 4-28 to the operating winding of slow-operate relay -Zfl. The duration of the prolonged pulse is suicient to operate slow-operate relay -Zl causing it to energize reset magnet -ll of selector switch 4I2 via conductor 4-29. This restores all selectors to their normal positions.

At the time when any selector switch is in its most advanced position, contacts li-Q and l-ti of relay #-135 serve as tone-muting contacts with respect to all subsequent tone pulses including the prolonged pulse. At the tenth selector switch position the circuit of loudspeaker ll-E includes contacts ll-'i, ll-l and is interrupted by each F opening of these contacts. Furthermore, since these are normally closed contacts, they ogen with the iirst movement of armature l-il-l thus minimizing the time during which each tone pulse will be audible.

indication the communication, a reset signal is transmitted from the central sta-v followed by a prolonged germer It will be observed that all be called simultaneously by dialing zero thereby causing the transmission of ten consecutive pulses or by some other single digitnurnber suiiicient to bring all selectors to their most advanced positions. Likewise, stations may be called in groups by dialing a number such as 19 which will call all stations whose code number has l for the first digit. Similarly, 23 will call all stations whose code numbers begin with 2.

Fig. 8 will now be considered which may be usedinstead of Fig. 5 with Fig. 4 when both audible and visible signals are desired. The operation of Fig. 4 remains unchanged. Upon establishing the circuit from ground at the iirst bank terminal through wiper arm A-l via conductors 4-33 and 6-33 to the winding of relay G-lll, this relay is energized from battery through resistor @-02 and establishes a locking circuit through armature 6-03 and stationary contact 6-94. This also lights the busy indicator lamp E-ll'l which is energized in multiple with the winding of relay G-lllA and resistor S-GZ. Armature E-t disengages from stationary contact G-l thereby interrupting the energization circuit for transmitreceive relay 6-l2 through switch G-ll. Thus far the operation of Fig. 6 corresponds with that of Fig. 5.

At the called station, selector switch 4|2 reaches its most advanced position and ground from back contact t-ll of relay 4-06 is applied via conductor il-2S and wiper last bank terminal to conductors l-SI and 6-31. Since relay G-l is locked in, this ground continues through armature 6435 and contact 6-08 of relay -ll via conductor 6-l3 to the winding of relay S-lll which is energized by battery passing through switchhook contacts -l via conductor G-IS. Relay S-ltl locks in through its armature E-l and contact -H. Call lamp G-IS lights by virtue of the ground maintained on conductor 643. It should be noted that almost simultaneously with the operation of relay S44, relay 643i releases. The release of relay 6-0! is slight' ground for relay E-Hl is applied upon the release of pulsing relay fand reaches relay G-l 4 sutilciently in advance of the release of relay G-EH to allow relay 6-14 to lock in through its own contacts S-IG and t-l'l. In connection with the use of Fig. 6 it should again be noted that the selector switch 4-12 must be of a type in which the wiper arms advance upon deenergization of the step magnet -IB. This action allows the operating ground for relay 644 to lock this relay in before the operating circuit is broken by the release of relay G-l accompanied by the opening ofcontacts 6-65 and 6-08. This is made possible by the slight delay in the release of relay B-lll produced by the short circuiting of its winding and also by the fact that relay G-Ili is of the fast acting type together with the action of the switch 4-l2 which applies the operating ground to relay 6-l4 and the releasing short circuit to relay 6-0l substantially simultaneously as it moves to its most advanced position.

With relay 6-[4 looked in and relay B-l released and with ground applied to conductor 6-3I at the called station through wiper arm 4-16 and mobile stations mayl press-to-talk arm -l through theA er 6-.2gt to call bell may be of any conventional type such as a thermal interruptor whichi has been, indicated byy way of illustration.

Switchhook E-i 6, is adapted to support a handset (not shown) used for two way communication. Upon seeing the call light and hearing the call bell, the operator lifts the handset from the switchhoolr fri-ld momentarily breaking the circuit of conductor .-lil. Because relay E44 is of the fast acting type, this short interruption of the circuit of its winding will cause it to open its holding contacts. 55,- E-I'l and release. This extinguishes the call light deis and silences. bell 6-22 by opening contacts; -Zl, 6 25. The accompanying closure of contacts 6-23, 6-24 connects the ground on conductor 549 through to the loudspeaker 4-42 and to the handset receiver 4-5I via conductors 6-34, 3 34.

ther use. Restoration ci the selector switch 4|2 at the called station removes the ground applied to conductors i-3l, -S and thus silences call bell 6-22. Because relay -Id is released by lifting the handset (not shown) from switchhoolr |6, it remains locked in maintaining call lamp S-IS in a lighted condition. Upon returning the operator will note that he has received a call by then call the central station operator to ascerm tain what was wanted. If the system is busy, he will necessarily be required to Wait, and this condition will be indicated by the busy lamp B-'l.

From the foregoing it will be apparent that a selective system of this type may be used for other purposes than communication. For eX- ample, this may be the selective control of various devices in either mobile or fixed outlying receiving stations. Such devices may comprise motors and the control may consist of the starting and stopping or reversing of the motors or the control of the The control may consist in the display of signals, similarly remotely selectively controlled, or in the case of weapons and the like it may consist in the remote guiding of a mobile mine and the control of the detonation of its explosive charge.

I have described what I believe to be the best embodiments of my invention. I do not Wish, however, to be oonned to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

l. A communication system comprising a radio transmitter and a plurality of outlying radio receivers to be selectively called by pulses from said transmitter, each of said receivers having means responsive to all pulses from said transmitter irrespective of the receiver to be called, means connected with said pulse responsive means to select said receiver for communication in response to a predetermined pulse code, means at said transmitter to transmit the pulse code of any desired receiver to be called, switch means at each of said receivers to prevent the utilization of communication currents received from said transmitter, and means to disable said preventing means at the receiver whose pulse code is the same as that received from said transmitter.

2. A communication system comprising a radio transmitter and a plurality Yof outlying radio re- I3: 6-22. The interrupter- SF2! ceivers, each of s id receivers having selective mit the selective call signals of the desired out lying receiver, said selective means; at each of said receivers being responsive to the 'selective cali signals therefor and also responsive to the selective call signals for each of saidl other oute lying receivers, a call signal indicator at each of;l said receivers responsive to said selective means. of said receiver in response to signals. corresponding to said station, but unresponsive tosignals not corresponding with. said station, and timing means comprising signal producing means at said transmitterto tran-smit to all said receivers a pre-restoring signal and a restoring signal in predetermined timed relation to each other to restore, all of said receivers to normal condition in unison with each other.

3. A communication system comprising a radio transmitter and a plurality of outlying radio receivers, each of said receivers having pulse responsive means connected therewith to select said receiver for communication in response t0 a predetermined pulse code, means at said transmitter to transmit the pulse code of the outlying receiver it is desired to call, said pulse responsive means at each receiver being responsive to the pulse code of said receiver to select said receiver and being responsive to the pulse code of the other receivers but incapable of selecting said receiver in response thereto, means at said transmitter to transmit a pre-restoring series of pulses of the type employed for calling a receiver, means to transmit a restoring pulse of a diiTerent type from said calling pulses in timed relation to said pre-restoring pulses, and means at each of said receivers for restoring said receivers in response to said pre-restoring and restoring pulses.

4. A communication system according to claim 3, having a switch, and means responsive to the operation of said switch to operate said pre-restoring pulse means and said restoring pulse means in predetermined timed relation to each other.

5. A communication system comprising a radio transmitter and a plurality of outlying radio receivers, a call signal and a busy signal at each of said receivers, means at said transmitter to transmit a selective call to a desired outlying receiver to operate the call signal at said receiver, means at said desired receiver to prevent the operation of the busy signal at said receiver, means at said other receivers to operate said busy signal in response to the selective call transmitted to said desired receiver, and means at each of said other receivers to 'prevent the operation of the call signal thereof in response to the selective call transmitted to said desired receiver.

6. A communication system comprising a central station and a plurality of outlying stations, each of said stations including a radio transmitter and a radio receiver, said receiver at each of said outlying stations having call responsive means connected therewith to select said receiver for communication in response to a predetermined call signal, means at said central station transmitter to transmit the call signal of the outlying station it is desired to call, means at the outlying station called by said central station to enable the transmitter of said called station to communicate with said central station, two call indicators at the receiver of at least one of said outlying stations, two of said call indicators being responsive to the predetermined call signal of said station to indicate when said station is called, means at said called station responsive to discontinuance of the call by the central station, to discontinue the operation of one of said two call signals, means to maintain the operation of the other of said two call signals Whensaidcallremains unanswered at the called station, said call responsive means at each of the uncalled outlying stations inoluding means to prevent communication over the transmitter thereof until said central station discontinues its call with said called station.

JACK KENDAL KULANSKY.

References Cited in the le of this patent UNITED STATES PATENTS Number aennevi Number Number Name Date Smythe et al. Feb. 9, 1926 Demarest et al. Oct. 23, 1928 Toison Dec. 9, 1930 Hershey Mar. 7, 1933 Fox Nov. 24, 1936 Von Der Linden Nov. 30, 1937 Kiner Dee. 26, 1939 Bowers Aug. 28, 1945 Newman Mar. 11, 1947 Neiswinter Aug. 19, 1947 Grove Jan. 24, 1950 Preston et al Mar. 21, 1950 Devaux June 27, 1950 FOREIGN PATENTS Country Date Great Britain July 5, 1945 

